How can I create a portal?
Here's a example I made a long time ago using the same techniques.

https://www.youtube.com/watch?v=Krn7CrYg4h8

This can be done easily with some render layer trickery.

Start out by putting both of your scenes on two separate layers, and align everything so that in your first scene, the opening into the next scene is in the same place.

Then, create an object on a trash layer, and shape it in the inside of your "portal". Then, in your renderlayer settings, in the mask group, select the layer you put your "portal mask".

Then, on your first layer, using the exclude group, exclude the layer with your first enviroment in it, and then do the oposite for the other one. This is done so that lighting and shadows from one enviroment, doesn't transfer over to the other.

Than, if your camera crosses into the other world, you need to stop the first one from rendering. This can be done by animating the checkbox next to the name of the layer.

After you render everything out, you can paste the two scene together using an alpha over node in the compositor.